The Button you see on the left is linked to the Dashboard and can be enabled to allow blocks editing, this means new blocks can be added to your own personal instance of the Dashboard and not affect any other user. This is also linked to the "Turn Editing On" button within Courses, this means that if you Turn the editing on you can also add blocks into Courses, whilst also enabling and disabling the "Customise this page", this 2 buttons are linked and perform the same accessibility to customisation on all sites.